Nightingale Reed-Warbler (Acrocephalus luscinia)

SPECIES OVERVIEW Description: The Nightingale Reed-warbler is characterized by a horn colored, long and slender bill and overall pale yellowish-buff color (Baker 1951; Pratt et al. 1987). The species is pale yellow-buff below, rufous-brown or grayish olive-brown above, with a pale yellow eyebrow (Baker 1951; Pratt et al. 1987). Head feathers are “shaggy” in appearance and often held erect and the tarsi and feet are light gray (Pratt et al. 1987). Females are similar to the males but are slightly smaller, while immature birds are similar to adults with duller and browner upper-parts and less yellow under-parts (Baker 1951).

Distribution: The Nightingale Reed-warbler is endemic to the Mariana archipelago and three subspecies are currently recognized: Acrocephalus luscinia luscinia, A. l. nijoi, and A. l. yamashinae (Mayr et al. 1986). The former, Acrocephalus luscinia luscinia, was originally found on Guam, Saipan, and Alamagan, but the Guam population was believed extirpated in the 1960s, the last known individual sighted at Agana Swamp in 1969 (Reichel et al. 1992). The species was likely extirpated from Guam by the introduced brown tree snake, with habitat loss and other introduced predators also serving as contributing factors (Reichel et al. 1992). In 2007, the Saipan population of Nightingale Reed-warblers was estimated at 2,742 (range = 1,686 – 3,956; Camp et al. 2009), a decline of 35% from the 1997 population estimate of 4,225 (USFWS 1998a), and 43% from 1982 estimate of 4,837 (Enbring et al. 1986). Camp et al. (2009) estimated that reed-warbler densities on the island have decreased by more than half from 1982 to 2007. Additionally, USFWS (1998a) found declines of reed-warbler numbers in developed parts of the Saipan and an increase in numbers in the more undeveloped northern areas of the island. However, reed-warblers are distributed over the entire island and are only absent from heavily developed or populated areas (DFW 2009; USFWS 1998b). Point –transect distance surveys for the reed-warbler on Alamagan in 2010 place the population at a mean of 946 individuals (95% CI 428 – 1,762; Marshall et al. 2011). CNMI DFW surveys of the island in 2000 estimated the population at a mean of 1,125 individuals (95% CI 504 – 1,539; Cruz et al. 2000a, Marshall et al. 2011). Super-typhoon Choi-wan passed directly over Alamagan on 15 September 2009, while super-typhoon Melor passed well to the south on of the island between 1 and 3 October 2009. These two storms, in addition to other super typhoons in the preceding decade, may have caused enough damage to have reduced the already small population size. Acrocephalus luscinia nijoi, the subspecies status of which USFWS (1998b) questions, has only been reported from Aguiguan and appears to have always been rare on that island (Reichel et al. 1992). After conducting surveys work in 1982, Enbring et al. (1986) reported a population of 4–15 birds on the island. Currently, Aguiguan’s population may be extirpated as no reed- warblers have been observed there since 1995 (USFWS 1998b), despite extensive surveys by CNMI DFW and USFWS in 2000 (Cruz et al. 2000b), 2002 (Esselstyn et al. 2002), and 2008 (USFWS unpublished data). The disappearance of reed-warblers from the island is probably due to a combination of feral goats, extensive clearing for agricultural development by the Japanese prior to World War Two, and the small size of the island (Enbring et al. 1986; Reichel et al. 1992). Acrocephalus luscinia yamshinae was only reported from the island of Pagan (Reichel et al. 1992). Currently, this population is presumed extinct as no sightings were reported in the 1970s, 1980s, and 2000 (Reichel et al. 1992, Cruz et al. 2000c). Habitat suitable for reed-warblers on Pagan had already been severely degraded by feral ungulates prior to a volcanic eruption in 1981, which destroyed the only known remaining habitat for the species on the island (Reichel et al. 1992; Cruz et al. 2000c). Surveys conducted in 2010 failed to detect the species (Marshall et al. 2011). In addition to these historical records, recent investigations on Tinian have revealed prehistoric evidence of the reed-warbler on that island (Steadman 1999). Given the current and prehistoric distribution of the reed-warbler in the archipelago it is possible that this species may have inhabited additional islands as well.

Habitat: The Nightingale Reed-warbler is common in upland and wetland habitats over most of Saipan (Reichel et al. 1992; USFWS 1998b). Upland habitats include tangantagan forests, tall elephant grass (Pennisetum polystachyon), bamboo, secondary forests, forest edges, and mosaics of these habitats (Craig 1992; Reichel et al. 1992, USFWS 1998b). USFWS (1998c) found reed- warbler densities highest in mixed secondary and tangatangan habitats in a survey of the Marpi Commonwealth Forest. Wetland habitats include native reed (Phragmites karka) marshes, marsh edges, mangroves, and wetland/upland econtones (Craig 1992; Reichel et al. 1992; USFWS 1998; Mosher 2006). Marshall (1949) reported reed-warblers in dense populations in marsh lands surrounding Lake Susupe, and marshes at Tanapag Harbor. Across Saipan, the reed-warbler tends to be absent from native limestone forest, mature secondary forests, beach strand, and swordgrass savannahs (Craig 1992; Reichel et al. 1992; USFWS 1998). On Alamagan Nightingale Reed-warblers have been found in habitats with partially open overstory and somewhat brushy understory, and in dense swordgrass (Miscanthus floridulus); however, none were observed in dense forest over 300 m elevation (Reichel et al. 1992; USFWS 1998b). Occupancy analysis of data collected on Alamagan by USFWS in 2010 indicated the reed-warblers occur in 19% of the island’s native forest, 49% of its coconut forest, and 69% its of secondary forest (Marshall et al. 2011). Cruz et al. (2000a) observed Nightingale Reed- warblers foraging throughout the mixed native and coconut forests, often in Hibiscus tiliaceus, and found them common in forested areas in the northern and southern parts of island, and at lower elevations; birds were not detected in open areas. These forested areas are among those most highly altered by human intervention, and are also highly impacted by feral animals. Reed- warblers were also heard in the remnant forests inside the volcanic crater. The bird’s use of both forest and open patches is similar to what is observed on Saipan (Cruz et al. 2000a). The USFWS (1998b) reported that reed-warblers on Aguiguan had inhabited formerly disturbed areas vegetated by groves of trees and thickets 1-2 meters tall. Craig and Chandran (1992) located Nightingale Reed-warblers in forest edges on the island and Engbring (1986) in native forest. Reichel et al. (1992) reported Nightingale Reed-warblers on Aguiguan using native forest with dense canopy. On Pagan, Nightingale Reed-warblers were reported to inhabit lake and wetland edge vegetation almost exclusively (Reichel et al. 1992; USFWS 1998b). On Guam, Nightingale Reed-warblers almost exclusively used Phragmites karka wetlands (Reichel et al. 1992; USFWS 1998), and less frequently tangantangan habitat (Reichel et al. 1992), and were most common at Agana swamp (Baker 1951). Food and Feed Habits: Data are limited concerning the foraging behavior of the Nightingale Reed-warbler. Baker (1951) reported that Seale (1901) found insects and larvae in the dissected stomachs of four reed-warblers. Likewise, Marshall (unpublished in USFWS 1998) also found coccinelid beetles and species’ of Hemiptera and Orthoptera in dissected reed-warbler stomachs. In the field, Marshall (1949) observed reed-warblers gleaning lizards, snails, spiders, and large insects from the ground. Mosher (2006) likewise observed nestlings being fed insects (ants, caterpillars, grasshoppers, moths, and praying mantids), spiders, and lizards (skinks and geckos). Craig (1992) noted the Nightingale Reed-warblers long bill relative to other reed-warbler species and postulated that this was related to a great range in food size and would be advantageous on a food-limited island. Craig (unpublished data as cited in USFWS 1998b) also reported that prey selection included insects of at least 3 cm in length and observed reed- warblers gleaning invertebrates from live and dead leaves and probing dead stubs. Mosher (2006) suggested that food does not appear to be a limiting factor for the species, labeling it as a generalist insectivore and carnivore. Behavior: The Nightingale Reed-warbler’s song, which may continue uninterrupted for several minutes, resembles that of American thrashers and mockingbirds, and includes trills, warbles, and whistles (Pratt 1987). Pratt (1987) described the species call is a loud “chuck.” Marshall (1949) reported reed-warblers mounting tree tops to sing in the evening, and singing constantly on moonlit nights. Males often sing in chorus at dawn, but singing is less frequent during the day (Marshall 1949). A raspy “ch-ch-ch” noise is often made by a breeding pair when around the nest (CNMI DFW unpublished data). Craig (1992) noted that the Nightingale Reed-warbler is extremely territorial and that it aggressively defends an approximate 1 hectare (9,338 ± 3,433 [SD] m2; n = 7) territory by singing from exposed treetops, interior thickets, or elephant grass stems. The species generally exhibits high site fidelity by males, and low site fidelity by females (Craig 1992). However, females will at times follow territorial males to defend a territory against an intruder (Craig 1992). Male defensive behavior includes song and pursuit, and two have been observed skirmishing on the ground (CNMI DFW unpublished data). Breeding: The Nightingale Reed-warbler exhibits a bi-modal profile in timing of breeding throughout a typical year. Mosher (2006) found that the occurrence of male reed-warblers with enlarged cloacal proturberances peaked in January and June, and active brood patches were observed in January, May, and June. Mosher (2006) also observed two peak nesting periods during his study on Saipan; January through March and July through September. Active nests were found in all months except November and December, with no nests being initiated after the month of September (Mosher 2006). Pairs that fledged young during a seasonal peak did not re- nest again until the next breeding peak. However, DFW (unpublished data, 2009) did observe a Nightingale Reed-warbler nest initiated in October. Unlike other species of reed-warbler, the Nightingale Reed-warbler is apparently monogamous (Mosher 2006, Craig 1992). Males defend their mates and nest sites, but not their feeding territories (Mosher 2006). Nesting: Unless otherwise cited, most available information on Nightingale Reed-warbler nesting comes from research conducted by Steve Mosher on Saipan (e.g., Mosher 2006, Mosher and Fancy 2002). Mosher (2006) states that information in Oustelet (in lit), Engbring et al. (1986), Craig (unpubl 1988) all generally fit his descriptions. Females constructed the nest with limited help from the male. Nest construction typically took 3-4 days and nests tended to be either tightly woven and compact or larger and tightly woven with bulky outer material. The nests were open cups, were circular to ovoid in shape, and some had an inner rim with an overhanging lip. Nightingale Reed-warblers appear to be opportunistic in selection of nesting material. Nests were typically composed of an outer structure of dried vine stems and tendrils of introduced bitter melon (Momordica charantia), wild passionfruit (Passiflora foetida) and of ironwood (Casauarina equisetifolia) branchlets. Nests in tangantangan (Leucaena leucocephala) forest had cup linings composed of tangantangan petioles, while those in mangrove forests were lined with more ironwood branchlets. Reed blades and spider web casing were also used. On Saipan, reed-warbler nests have been found in native trees including Ochrosia mariannensis, Hibiscus tiliaceus, Bruguiera gymnorrhiza, and Casauarina equisetifolia, introduced tree species including Leucaena leucocephala and Pithecellobium dulce, and in native reed (Phragmites karka) wetlands. Nests have also been found by CNMI DFW biologists in stands of the tall, introduced grass Pennisetum polystachyon and in Melanolepis multiglandulosa (CNMI DFW unpublished data). Mosher (2006) did not find any Nightingale Reed-warbler nests specifically in limestone forest on Saipan. Nests placed in Leucaena leucocephala were generally attached to the main stem/trunk and several lateral branches with at least one branch supporting the nest’s bottom. Those in Ochrosia mariannensis were supported by the main truck and 3-5 branches while those in Casauarina equisetifolia were found in high drooping branches that extended away from the main trunk. Nests observed in Phragmites karka wetlands were usually supported by three vertical reed stems and two leaning stems. In all cases there was always a branch or stem supporting the base of the nest, which was usually woven in place by vines. Reed-warblers appear to have a principal nesting area within a given territory, with nests from previous attempts often in close proximity to each other. No nests were observed to be used more than once, but reed-warblers were observed building new nests on top of old nest structures. Re-nesting often does occur after the initial nest fails. Eggs, Incubation, Hatching, Growth, and Development: Information here on nesting ecology comes from Mosher’s work on Saipan, data for which can be found in Mosher (2006) and Mosher and Fancy (2002). Nightingale Reed-warbler eggs are sub-elliptical in shape, dull white to cream to ivory buff in color. They are spotted, speckled, and blotched with irregular shaped markings over entire shell, typically with overlapping markings around broader end of the egg. The markings are gray, brown, black, and rust in color with pinpoints up to 2.3mm in diameter. Mean egg length was 23 mm (range = 21–25.8 mm), mean egg width 16.9 mm (range = 15.9–18 mm), and mean egg mass was 3.1g (range = 2–3.8 g); mean clutch size was 2.5 (range = 2–4 eggs), with a mode of two. Brood size ranged from one to three nestlings with a mode of two. The mean nesting period is 32.5 days +/-1.7 (range = 31–34 days). Eggs are laid on successive days and incubation ranges from 15–17 days. Both male and female incubate but only females develop a brood patch (also observed by CNMI DFW, unpublished data). Nests (n = 22) observed by Mosher (2006) exhibited a success rate of 44% over two years. Nightingale Reed-warbler nestlings hatch with closed eyelids, dark gray to black skin, no down, and bright yellow gape flanges. Nestlings are fed on average 7.7 times per hour and adults remove fecal sacs from nest. Prior to fledging, nestlings are almost completely feathered except around eyes, ears, chin, and throat; the egg tooth is present on the bill two to three days prior to fledging. Fledging typically occurs 15 to19 days post-hatching and weak flying young leave the nest with poorly developed flight feathers and short tails (also observed by CNMI DFW, unpublished data). At approximately 14 days, the mean mass of nestlings is 28.4g (range = 24–35g), mean wing chord is 50.3 mm (range = 39–58mm), mean exposed culmen is 17.7 mm (range = 15.1–22.9mm), mean culmen nares-to-tip is 10.4 mm (range = 8.4–11.6mm), and mean tail length is 13.3 mm (range = 4–20.5mm). The mean number of tail feathers at 14 days is 10.2 (range = 9–12), with mode of 10. Juvenile plumage is brown with buff edges, and the breast, belly, vent, thighs, and undertail coverts light yellow to cream, with flanks brownish yellow to buff. Juveniles lack the pale yellow supercilium and black lores that are characteristic of adults. Threats: Threats to the Nightingale Reed-warbler include habitat loss, habitat degradation from feral ungulates, introduced predators (including the Brown Tree Snake), and the spread of invasive species (USFWS 1998b; Berger 2005). Habitat loss on Saipan is due to conversion of forested habitat for agriculture, homesteads and other residential development, and tourist-related facilities. Because of development pressures over the last two decades, suitable habitat for the reed-warbler on Saipan has declined (USFWS 1998b, CNMI DFW unpublished data). Habitat loss is also caused by the spread of non-native vines, especially scarlet gourd (Coccinia grandis; Berger 2005). Non-native vines can quickly cover forested areas, shading the trees from light and causing the forest to die. Feral ungulates, goats in particular, have degraded suitable Nightingale Reed-warbler habitat on Aguiguan and Alamagan. Continued grazing on Aguiguan could limit the opportunities for reed-warblers to recolonize or be introduced to the island, while grazing damage on Alamagan may be a cause of reed-warbler population decline there. Habitat loss due to feral ungulates on Alamagan is a severe problem, and could lead to extinction or decline of reed-warblers on the island much as it did on Pagan (Cruz et al. 2000a). Introduced predators including cats (Felis catus), rats (Rattus spp.), monitor lizards (Varanus indicus), and brown treesnakes (Boiga irregularis) also play a significant role in reed-warbler population declines. Currently, the brown treesnake is the most serious predation threat to the reed-warbler on Saipan. This species of snake caused the extinction or severe decline of most native birds on Guam (including the reed-warbler) within 35-40 years after its accidental introduction (Reichel et al. 1992). Mosher (2006) found that predation caused nest failure in 75% of failed nests. Predation occurred primarily by rats (71%), cats (5%) and unknown predators (24%). Monitor lizards and collared kingfishers may also predate reed-warbler nests. Camp et al. (2009) found that insectivorous birds such as the reed-warbler were declining on Saipan, indicating that prey availability may a limiting factor for the species population on the island. Research Needed: Information on the response of Nightingale Reed-warblers to development and disturbance to aid in development of appropriate avoidance and minimization measures and mitigation. Research on population dynamics and taxonomy and Population Viability Analysis (PVA) for the population on Saipan. Conservation Recommendation: The USFWS (1998b) and Berger (2005) provided a number of conservation recommendations including:

Interdiction of the brown tree snake

Feral ungulate control on Aguiguan and Alamagan

Predator control, especially around nests and in conservation areas

Eradication of rats, cats, and other predators from Aguiguan

Establishment of a captive breeding program and translocation of birds to northern islands

Habitat protection through avoidance and minimization measures and mitigation placed on • development projects